Chromosome Fusion

Chimps have 48 chromosomes, humans only 46. How can they be closely related?

Despite the difference in chromosomes, chimp DNA is nevertheless closer to human DNA than it is to gorilla DNA.


One Fusion Event

First of we need to realise the chromosomes are paired, so this is a jump from 24 pairs to 23 pairs. 

What we are talking about is two pairs of chromosomes fusing - joining end to end - to produce one pair. It is like having two sections of string, tying the right end of one to the left end of the other, and ending up with one string approximately as long as the two originals combined.

Imagine three consecutive strings:

abcdefg + hijklmnop + qrstuvwxyz

This is analogous to the ancestor with 24 chromosome pairs, but just looking at three of them. Some mutation happens, and joins two string together. Now there are only two consecutive strings:

abcdef@hijklmnop + qrstuvwxyz

This is analogous to the ancestor with 23 chromosomepairs, but just looking at two of them.

If you look carefully you will see the DNA is virtually unchanged. The difference we are talking about here is pretty slight.

That is the simplified version. What actually happened?

Human chromosome 2 was formed by the head-to-head fusion of two ancestral chromosomes that remained separate in other primates. Sequences that once resided near the ends of the ancestral chromosomes are now interstitially located in 2q13–2q14.1. Portions of these sequences had duplicated to other locations prior to the fusion. [B]Here we present analyses of the genomic structure and evolutionary history of >600 kb surrounding the fusion site and closely related sequences on other human chromosomes.[/B] Sequence blocks that closely flank the inverted arrays of degenerate telomere repeats marking the fusion site are duplicated at many, primarily subtelomeric, locations.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC187548/


Breeding

How does that affect breeding? One parent will donate abcdef@hijklmnop + qrstuvwxyz and the other donates abcdefg + hijklmnop + qrstuvwxyz. Not a problem because the actual DNA is virtually the same.

To put this in perspective, consider the X and the Y chromosomes in humans. They are part of the chromosome pair in males, but are very mismatched; the Y chromosome is only about a third the size of the X chromosome. Women have two sex chromosomes that are the same length, men have one that is three times the size of the other!

When comparing chimp and human DNA, chimps have two chromosomes that correspond to just one in humans, but they are nevertheless far more a like that the X and Y chromosomes.

Also worth noting that while 46 is the normal number of chromosomes in humans, some people have fewer and some people have more. This is usually associated with a disease, but nevertheless it shows that the divide between 46 and 48 is not as you might think.


Examples of different numbers of chromosomes

Butterflies and moths have wildly different numbers of chromosomes, and, at least in some instances, that is no barrier to reproduction.

The order Lepidoptera, which comprises more than 160 000 described species of butterflies and moths, is one of the most speciose branches of the tree of life. Its remarkable diversity is accompanied by a tremendous variation in chromosome numbers, [B]ranging from 5 to 223 chromosomes[/B] in the haploid karyotype [1,2]. .... Closely related species with different chromosome numbers can often be crossed [8,9] and [B]hybrid fitness may not necessarily be reduced[/B] [10,11].

https://royalsocietypublishing.org/doi/10.1098/rstb.2019.0539


Some mammals are similar.

Brocket deer (Mazama) have impressive chromosomal variation, with diploid numbers ranging from 2n = 32 to 2n = 70

https://www.redalyc.org/journal/457/45765132012/html/

More on them here:

https://bmcecolevol.biomedcentral.com/articles/10.1186/1471-2148-14-40

Presumably in the creationist model, all that variation evolved after the flood, right? Otherwise Noah would have had to have a pair of Brocket deer with 32 chromosomes, a pair with 34, a pair with 36, and so on. So it turns out that creationism requires for evolution with changing numbers of chromosomes.


Also of note are B chromosomes, which are additional chromosomes some individuals have.

In addition to the normal karyotype, wild populations of many animal, plant, and fungi species contain B chromosomes (also known as supernumerary, accessory, (conditionally-)dispensable, or lineage-specific chromosomes).[1] By definition, these chromosomes are not essential for the life of a species, and are lacking in some (usually most) of the individuals. Thus a population would consist of individuals with 0, 1, 2, 3 (etc.) B chromosomes.[1] B chromosomes are distinct from marker chromosomes or additional copies of normal chromosomes as they occur in trisomies.

https://en.wikipedia.org/wiki/B_chromosome


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